In the casting of ferrous and non ferrous metal into parts, the cast part is formed by pouring the molten ferrous or non ferrous metal into a mold. When the part has internal openings or paths, sand cores are made using foundry sand and a binder to the shape of the internal openings or paths, and are positioned in the proper location in the mold. The molten metal is poured into the volume between the mold and the core(s) usually surrounding some or most of the core. When the metal solidifies, the mold is opened and the part is removed. In most cases, the core(s) remain in the interior regions its presence has formed and must be removed.
U.S. Pat. No. 5,423,370, the disclosure of which is incorporated herein by reference, describes the invention of a fluid bed furnace for the removal of sand cores from castings, employing a thermal process based on the use of fluidized sand of the same type as used to make the sand core. This same patent describes the use of the fluid bed furnace for the heat treating of the aluminum castings.
In the case of ferrous and non ferrous metal parts formed by other methods than casting, the fluid bed has been established as an important processing approach for the heat treating and cleaning of parts and other objects in significant commercial applications. This is exemplified by U.S. Pat. Nos. 4,512,821; 4,524,957; and, 4,547,228, the disclosures of which are incorporated herein by reference.
There are a number of known techniques to provide energy input to a fluid bed furnace to achieve the required temperature level of the fluidized solids bed and meet the energy requirement of the specific process being performed, plus the heat losses of the system. The source of energy input to a fluid bed furnace system is typically electricity or fuel such as natural gas or oil.
The mechanism of transferring the energy from an energy source to the fluidized solids bed is typically accomplished by one or a combination of the following methods:    Mechanism i: Heating the fluidizing gas phase before entering the furnace to a temperature above the operating temperature of the fluidized solids bed, as shown in FIG. 1. When the high temperature fluidizing gas enters the fluid bed through the fluidizing gas distribution tuyeres, it provides the required energy input. This is termed “direct heating”.    Mechanism ii: Transferring energy through heat transfer surfaces in contact with the fluidized solids bed, typically through heating tubes submerged in the fluidized solids bed, or through the walls of the vessel housing the fluidized solids bed from a heating mantle surrounding the walls, as shown in FIG. 2. This mechanism of energy input is termed “indirect heating”.    Mechanism iii: Direct injection of fuel into the fluidized solids bed in gaseous, liquid or solids form and combusting the fuel while it is within the fluidized solids bed; i.e., below the top level of the fluidized solids bed, as shown in FIG. 3.
The choice of energy source, is typically economically driven. The choice of mechanism of energy transfer to the fluid bed typically depends upon the geometric configuration of the furnace and the characteristics of the process application involved. This choice is typically determined by the gas phase environment required by the submerged parts.
In applications where the products being process cannot be contacted by products of combustion of typical fuels, the mechanism of transferring energy to the fluid bed must be limited to indirect heating of the fluid bed by Mechanism ii, and/or indirect heating of the fluidizing gas to elevate its temperature followed by direct heating of the fluid bed by Mechanism i.
In these cases, direct injection of fuel into the fluidized solids bed by Mechanism iii, cannot be employed due to combustion gases being present in the fluidized solids bed which has an adverse effect on the quality of the products.
In cases where the combustion products of the typical fuels can contact the parts without quality degradation, and the operating temperature of the fluidized solids in the furnace is higher than the ignition temperature of the fuel, so there is no concern about ensuring complete combustion of the fuel in the fluidized bed of solids, economical considerations generally favor Mechanism iii, above as shown in FIG. 3. In FIG. 3, the unit is shown equipped with both direct fuel injection and direct combustion air injection. In situations where the fluidizing gas is air, which is the case in many important commercial applications, the direct injection of combustion air is not required because the fluidizing air provides the necessary oxygen for combustion. It is only necessary to feed the fuel to the fluidized bed.
In most cases involving heat treating of metal parts, it is required to maintain careful control of the composition of the fluidizing gas. This requirement typically eliminates Mechanism iii, above from consideration for these applications.
For the very important application to sand core debonding of aluminum castings and heat treating aluminum castings and other aluminum parts, the processes take place at approximately 550° C. This temperature is below the ignition point of natural gas and other fuels so the use of Mechanism iii, is frequently eliminated based on safety concerns and/or the costs involved in protection devices to practice Mechanism iii, safely.
This typically limits consideration to Mechanisms i and ii, above for the important commercial applications involving processing of aluminum castings and other aluminum parts and heat treating of metals.
Mechanism i, is generally the lower cost approach to providing the required energy to the fluidized solids bed using a fluidizing gas heater to elevate the temperature of the fluidizing gas. The maximum rate of energy transfer to the gas fluidized solids bed possible by this mechanism, is limited by the maximum temperature the furnace fluidizing gas distribution tuyere system can withstand mechanically, and the maximum fluidizing velocity that can be applied to the solids being fluidized without excessive entrainment of solids in the fluidizing gas exiting from the furnace.
The temperature of the fluidizing gas is typically elevated using a gas heater, and then feeding the high temperature fluidizing gas through the distribution tuyeres of the bed, as shown in FIG. 1. The fluidizing gas heater can be either direct fuel fired when the products of combustion are acceptable in the gas phase of the fluidized solids, or indirectly heated by fuel or electricity when the application cannot accept products of combustion in the fluidizing gas phase.
The primary disadvantage to the use of Mechanism i, is that in applications requiring high rates of energy input to the gas fluidized solids, the temperature of the fluidizing gas must be significantly higher than the temperature of the fluidized solids bed.
This high temperature fluidizing gas elevates the temperature of the fluidized solids in the immediate vicinity of the fluidizing gas discharge tuyeres well above average bed temperature. This high temperature can in some cases, damage the parts being processed if the parts come close to, or contact a tuyere.
As an example, for the case of processing aluminum metal parts, a typical fluid bed furnace might be solution annealing the parts at 500° C. in the bed of fluidizing solids with the fluidizing gas temperature at approximately 815° C. If an aluminum part comes in contact or in the close vicinity of a fluidizing gas tuyere, the part can be melted or seriously distorted. In addition, there are typically small shavings, pieces, or chips of aluminum which fall from the parts being processed which find their way to the bottom of the fluid bed furnace and gradually accumulate over a period of time. When these pieces approach the vicinity of a tuyere, or contact a tuyere, they are usually melted and gradually surround the tuyeres and impede the flow of air.
The improved fluidizing gas distributor of this invention reduces the temperature of the fluidizing gas before it discharges through the tuyeres, thereby eliminating the local high temperature regions in the vicinity of the tuyere and eliminates the problem of melting or distorting the parts in the vicinity of the tuyeres.
This invention is a new improved approach to transferring energy into a fluidized bed and can benefit applications that can or cannot accept products of combustion of the energy source in the fluidization gas phase, and whether or not the temperature of the fluidized solids bed is above or below the ignition temperature of the fuel used as the energy source.
It accomplishes this broad application advantage by combining some of the concepts of Mechanism i and Mechanism ii, in an innovative arrangement of heating the fluidized solids by indirect heat transfer followed by direct heating by the fluidizing gas discharging from the gas distribution arrangement. This configuration can be particularly favorable for heat treating metal parts, cleaning metal parts, removing sand cores and enclosing molds from castings, but is also advantageous in some fluid bed reactor configurations involving fluid bed furnaces.